This invention relates to an improved method of forming a silicon oxide film on a substrate by plasma-enhanced chemical vapor deposition (CVD) using an organosilicon compound as the silicon source and apparatus for the improved CVD method.
In the fabrication of semiconductor devices, a widely employed technique for forming a silicon oxide film on a substrate is CVD using the reaction between a silicon source gas and oxygen gas. For this purpose a conventional silicon source gas is a silane, but recently attention has been devoted to the use of an organosilicon compound such as tetraethoxysilane in view of, in particular, improved step coverage of the film deposited on a substrate surface having steps such as aluminum wiring lines.
In using tetraethoxysilane it is known to use ozone, as an oxygen gas containing about 1-10% of ozone, in order to accomplish CVD at relatively low temperatures and also to increase the deposition rate. Also it is known to employ plasma-enhanced CVD (herein, plasma CVD for brevity) in order to further increase the deposition rate and enhance the quality of the deposited films. However, on a substrate having steps the film deposited by plasma CVD is insufficient in step coverage and cannot always fill spaces between the steps. In the case of narrow spaces with aspect ratios greater than 1, often voids remain in the plasma CVD film filling the spaces.
As a remedy for inferior step coverage of the plasma CVD film, it has been tried to alternate plasma CVD and plain heat-initiated CVD (herein, thermal CVD for brevity) using tetraethoxysilane and ozone. A silicon oxide film formed by this method has a multilayer structure, wherein each layer formed by thermal CVD is fairly good in step coverage and gap filling capability and hence compensates the inferior step coverage of the adjacent layers formed by plasma CVD. However, in this multilayer film the layers formed by thermal CVD do not possess good properties of plasma CVD films and often contain considerable moisture. Therefore, in the case of using the multilayer film as a dielectric film in a semiconductor device with multilayer interconnections there are possibilities of defects such as interlayer peeling and bad contacts in through-holes formed in the dielectric film. To solve these problems it is effective to considerably increase the ozone concentration in the thermal CVD process, but a disadvantage of this measure is that step coverage of the thermal CVD layers becomes inferior.
In forming a silicon oxide film as an interlayer dielectric film in a semiconductor device with multilayer interconnections, another technique to compensate insufficient step coverage and gap filling capability of a plasma CVD film is the application of a silica dispersion liquid. Initially a plasma CVD film is deposited on a substrate formed with aluminum wiring lines to a thickness limited so as not to leave voids in the film in the spaces between the wiring lines. Then a silica dispersion liquid is applied onto the plasma CVD film, followed by a heat treatment at a relatively low termperature for evaporating the solvent and another heat treatment at a higher temperature for enhancing the quality of the silica film. The application of the silica dispersion liquid and the heat treatments are repeated until the silica film surface becomes nearly flat over the aluminum lines and spaces between the lines. Then the silica film is planarized by a reactive ion etching method. If the plasma CVD film on the aluminum lines is exposed by this etchback operation there arises a local increase in the etch rate of the silica film by the action of oxygen supplied from the plasma CVD film, and hence the etched silica film surface becomes dented in areas over the spaces between aluminum lines. Finally another plasma CVD film is deposited on the planarized silica film. This method needs complicated operations and tends to suffer from low yield.